It is known that skin gets thinner and reduces its metabolism with aging.
While advancing years are a macroscopically significant factor causing cutaneous aging, influences of dryness, oxidation, and ultraviolet (UV) rays of sunlight are believed to be causes which directly attribute to cutaneous aging.
Various means for inhibiting the skin from being aged have hitherto been taken. For example, (1) preparations for external use containing various humectants for inhibiting cutaneous aging caused by dryness, (2) preparations for external use containing antioxidants for inhibiting cutaneous aging caused by oxidation, such as vitamin E, and (3) preparations for external use containing ultraviolet absorbers for inhibiting cutaneous aging caused by exposure to UV light have been proposed.
However, all these means conventionally taken for prevention of cutaneous aging are based on efforts to minimize the above-mentioned external factors which, as a result, relate to cutaneous aging and are no better than a kind of symptomatic treatment. There is no denying that the effects produced are insufficient. Hence, it has been demanded to establish a means of radical treatment for stopping or inhibiting a phenomenon which accelerates cutaneous aging more directly.
Increase of collagen cross-linking mainly in the dermis is known as a phenomenon which accelerates cutaneous aging more directly (see Albert M. Kligman and Yoshio Takasu (ed.), Cutaneous Aging, pp. 263-274, University of Tokyo Pres. (1988) and Sugiyama T.; Fujimoto D., Arai C. and Hasegawa M., Biomed. Res., Vol. 8, pp. 349-351 (1987)).
That is, the above-mentioned factors of cutaneous aging, especially exposure to long-wavelength UV rays (hereinafter referred to as UV-A) easily reaching to the dermis, are accompanied by reductions in growth activity of fibroblasts which are main cells in the dermis or functions of metabolic turnover (synthesis and decomposition) of collagen, etc., and thereby reduction in turnover rate of collagen, etc. With respect to UV-induced collagen cross-linking, reference can be made in Fujimori E., FEBS Lett., Vol. 235 (1-2), pp. 98-102 (1988). It seems to follow that collagen, etc. undergo various modifications or denaturation, resulting in increased cross-linking of collagen in the skin.
As collagen cross-linking proceeds, the skin loses its elasticity, and wrinkles and sagging increase, and cutaneous aging thus proceeds.